U.S. patent number 5,173,909 [Application Number 07/720,691] was granted by the patent office on 1992-12-22 for wavelength tunable laser diode.
This patent grant is currently assigned to Hitachi, Ltd.. Invention is credited to Naoki Chinone, Akihiko Oka, Katutoshi Saito, Shinji Sakano.
United States Patent |
5,173,909 |
Sakano , et al. |
December 22, 1992 |
**Please see images for:
( Certificate of Correction ) ** |
Wavelength tunable laser diode
Abstract
A wavelength tunable laser diode comprising a temperature
variable heater separated from an active layer by a distance less
than the thickness of a compound semiconductor substrate. Because
the heater is located very close to the active layer, the response
time of temperature change is improved. That in turn widens the
tunable range of the laser diode.
Inventors: |
Sakano; Shinji (Hachiohji,
JP), Oka; Akihiko (Musashino, JP), Saito;
Katutoshi (Higashi-Yamato, JP), Chinone; Naoki
(Chofu, JP) |
Assignee: |
Hitachi, Ltd. (Tokyo,
JP)
|
Family
ID: |
16151205 |
Appl.
No.: |
07/720,691 |
Filed: |
June 25, 1991 |
Foreign Application Priority Data
|
|
|
|
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Jul 13, 1990 [JP] |
|
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2-184316 |
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Current U.S.
Class: |
372/20; 372/34;
372/36 |
Current CPC
Class: |
H01S
5/04254 (20190801); B82Y 20/00 (20130101); H01S
5/0612 (20130101); H01S 5/0622 (20130101); H01S
5/34313 (20130101); H01S 5/06206 (20130101); H01S
5/34306 (20130101); H01S 5/06255 (20130101); H01S
5/0261 (20130101) |
Current International
Class: |
H01S
5/042 (20060101); H01S 5/06 (20060101); H01S
5/00 (20060101); H01S 5/343 (20060101); H01S
5/026 (20060101); H01S 5/062 (20060101); H01S
5/0625 (20060101); H01S 003/10 () |
Field of
Search: |
;372/20,34,36 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Electronics Letters, Jul. 20, 1989, vol. 25, No. 15, pp. 990-991.
.
Electronics Letters, Jan. 4, 1990, vol. 26, No. 1, pp.
46-47..
|
Primary Examiner: Epps; Georgia Y.
Attorney, Agent or Firm: Kenyon & Kenyon
Claims
What is claimed is:
1. A wavelength tunable laser diode comprising at least:
a compound semiconductor substrate having a first conductivity
type,
an active layer formed on the surface of said compound
semiconductor substrate, said active layer being made of a first
compound semiconductor layer with a band gap energy level lower
than that a said compound semiconductor substrate,
a second compound semiconductor layer formed on said active layer
and having a second conductivity type opposite to said first
conductivity type,
a third compound semiconductor layer formed on said compound
semiconductor substrate and adjacent to said active layer and said
second compound semiconductor layer,
a first electrode disposed on a predetermined portion of said third
compound semiconductor layer and electrically connected to the
surface of said second compound semiconductor layer,
a temperature variable heater disposed above said active layer and
separated from said active layer by a distance less than the
thickness of said compound semiconductor substrate,
at least one second electrode electrically connected to said
temperature variable heater and formed above said third compound
semiconductor layer, and
a third electrode formed on a rear surface of said compound
semiconductor substrate.
2. A wavelength tunable laser diode according to claim 1, wherein
said temperature variable heater is deposited above said second
compound semiconductor layer with said first electrode and said at
least one second electrode and an insulation film interposed
therebetween.
3. A wavelength tunable laser diode according to claim 2, wherein
said temperature variable heater is constituted by a resistance
film.
4. A wavelength tunable laser diode according to claim 3, wherein
the width of said resistance film is less than that of said
compound semiconductor substrate.
5. A wavelength tunable laser diode according to claim 3, wherein
said resistance film is made of a platinum.
6. A wavelength tunable laser diode according to claim 3, wherein
said resistance film is connected to said at least one second
electrode and further comprising a second insulation film deposited
on said third compound semiconductor layer, said at least one
second electrode being deposited on said second insulation
film.
7. A wavelength tunable laser diode according to claim 2, wherein
said temperature variable heater is a Peltier device.
8. A wavelength tunable laser diode according to claim 1, wherein
said compound semiconductor substrate is constituted by an InP
layer.
9. A wavelength tunable laser diode according to claim 1, wherein
said active layer has a multiple quantum well structure of
GaAs/InGaAsP.
10. A wavelength tunable laser diode according to claim 1, wherein
said second compound semiconductor layer is constituted by an InP
layer.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a wavelength tunable laser diode
and, more particularly, to a wavelength tunable laser diode
especially suitable for use with coherent optical communication
systems.
Wavelength-division-multiplexing communication involves having a
light beam of multiple wavelength carry signals so as to transmit
data of multiple channels over a single optical fiber cable. Thus
an increased transmission capacity is made available with this
method. In this case, the receiver side requires installing a laser
diode for a local oscillator. The laser diode allows the receiver
to tune in through interference to signals of a specific wavelength
out of the received wavelength-division-multiplexed light.
Generally, a wavelength tunable laser diode is used as the laser
diode for the local oscillator. The wavelength tunable laser diode
permits selection of signals of any specific wavelength out of a
plurality of wavelengths in the received light. This laser diode
must meet three requirements: (1) the tunable range must be wide;
(2) the spectral line width must be narrow enough to permit
utilization of interference; and (3) the optical output power must
be high so as to improve the S/N ratio for the signal.
A first example of the prior art wavelength tunable laser diode is
one described in "Electronics Letters," Vol. 25, No. 15 (1989), pp.
990-991. With a grating arrangement inside, this laser diode is a
distributed feedback laser diode having a device length of 1.2 mm.
The large device length allows the electrode arrangement to be
divided into a plurality of electrodes. Varying the injecting
current ratio between the multiple electrodes provides a laser beam
having a tunable range of 2.2 nm, an optical output power of 20 mW
or more and a spectral line width of 1 MHz or less.
A second example of the prior art wavelength tunable laser diode is
one described in "Electronics Letters," Vol. 26, No. 1 (1990), pp.
46-47. This is a distributed feedback laser diode with a waveguide
inserted therein, the waveguide varying the refractive index. The
laser diode of this structure provides a tunable range of 7 nm
while its optical output is as low as 4 mW or less.
A third example of the prior art wavelength tunable laser diode is
one that utilizes one known laser diode characteristic: that the
wavelength typically varies at a rate of 0.1 nm/.degree.C. The
laser diode of the above characteristic is attached to a Peltier
device used for temperature control. As the temperature of the
laser diode is varied, the wavelength emitted therefrom is
controlled accordingly.
With the first prior art example comprising multiple electrodes,
the reflecting band width of 3 nm for the internal grating limits
the tunable range to about 3 nm. Given that constraint, it is
difficult to widen the tunable range to 4 nm or more.
With the second prior art example incorporating the
refractive-index-tuning waveguide, the carrier doped to vary the
refractive index worsens the optical output power through
absorption. Given that kind of laser diode, it is impossible to
obtain an optical output of 10 mW or higher.
With the third prior art example that uses the Peltier device for
laser diode temperature control, the response time of temperature
change is prolonged. That is, wavelengths are switched in units of
as long as seconds. This is because the Peltier device is attached
to the back of the substrate detached from an active layer of the
laser diode, with the result that the temperature of the whole
laser diode can only be controlled from the back of the
substrate.
As described, the conventional wavelength tunable laser diodes have
yet to fulfill in practical terms the requirements of output,
tunable range and response time at the same time. There has been an
urgent need to develope a laser diode that would meet all such
requirements.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to overcome the
above-described drawbacks and disadvantages and to provide a
wavelength tunable laser diode which provides a high optical output
power, a wide tunable range and a high wavelength switching
speed.
It is another object of the present invention to provide a
practical wavelength tunable laser diode which provides an optical
output power of 10 mW or higher, a spectral line width of 10 MHz or
less, a tunable range of 4 nm or more, and a response time on the
order of 10.sup.-3 seconds (1ms) or less.
In achieving the foregoing and other objects of the invention and
according to one aspect thereof, there is provided a wavelength
tunable laser diode comprising a semiconductor substrate, an active
layer and a temperature variable heater, wherein the temperature
variable header is separated from the active layer by a distance
less than the thickness of the semiconductor substrate, the
temperature of the heater being controlled so as to vary the
wavelength of the output light beam from the laser diode.
According to another aspect of the invention, there is provided a
wavelength tunable laser diode comprising at least: a semiconductor
substrate; a first semiconductor layer formed on the semiconductor
substrate and constituting an active layer whose band gap energy is
less than that of the semiconductor substrate; a second
semiconductor layer formed on the semiconductor substrate and
having the polarity type opposite to that of the semiconductor
substrate; and electrodes formed above the second semiconductor
layer and under the first semiconductor layer, the electrodes being
used for carrier injection; wherein a resistance film is deposited
as a temperature variable heater on the electrode above the second
semiconductor layer with an insulation film interposed between the
film and the electrode. This constitution provides very favorable
results.
According to the invention, the temperature variable heater is
located near the active layer. This means that the temperature
around the active layer is varied at a high rate. In particular,
the heater may be deposited on the upper-surface electrode opposite
to the semiconductor substrate facing the active layer. In that
case, there exists no thick semiconductor substrate having a high
thermal capacity between the heater and the active layer. This
structure makes the response time of temperature change around the
active layer shorter than ever before as the temperature of the
heater is varied. When the temperature around the active layer
changes, the refractive index around the active layer varies
correspondingly. Where the oscillation wavelength of the laser
diode is 1.55 .mu.m, the wavelength thereof varies at a rate of 0.1
nm/.degree.C. Thus if the temperature around an active layer 1 in
FIG. 1 is changed by 40.degree. C., a tunable oscillation
wavelength range of approximately 4 nm is made available.
When the resistance film is used as the heater located on the
device surface very close to the active layer, the distance between
the heater and the active layer is as short as several .mu.m,
compared with about 200 .mu.m, i.e., the typical thickness of the
prior art semiconductor substrate. Thus controlling the heater
temperature with the wavelength tunable laser diode of the
invention makes the response time of temperature change near the
active layer significantly shorter than conventional
temperature-controlled wavelength tunable laser diodes.
FIGS. 2a graphically shows how the response time varies when
temperature is varied from 20.degree. C. to 60.degree. C. on a 200
.mu.m thick InP substrate. A curve 10 plots response time
measurements taken when the heater is located at the back of the
substrate, and another curve 11 plots response time measurements in
effect when the heater is provided over the entire substrate
surface.
When temperature is controlled from the back of the substrate, the
heat dissipates throughout the substrate and thus a time delay of
about 0.03 ms is experienced, as indicated by the curve 10. When
the heater is provided over the entire surface of the substrate
whose back is kept to a constant 20.degree. C., no time delay
occurs; the response is immediate.
If the resistance film deposited as the heater over the substrate
surface is made substantially the same in width as the active
layer, the response time is further shortened. Because the active
layer is formed near the surface where the resistance film is
deposited as the heater, making the heater width the same as that
of the active layer allows the temperature of the active layer 1 to
be raised with very little heat dissipation. This setup makes the
width of the heater less than if the entire surface of the
substrate is heated. Since the change in temperature is dependent
on the power density per unit area, the power density is raised
when the heater width is reduced. The raised power density leads to
higher energy efficiency, which in turn improves the response time
of temperature change significantly.
Referring to FIGS. 2b, a curve 13 plots response time measurements
taken when the heater is provided over the entire width of an InP
substrate 400 .mu.m wide; another curve 12 plots response time
measurements taken when a heater 10 .mu.m wide is provided on the
surface of the InP substrate 400 .mu.m wide. In either case, the
heater is deposited on the surface of the InP substrate. The
graphic representation of FIG. 2B shows that when the heater is
made smaller in width than the substrate, the power density is
boosted. In turn, the raised power density makes the response time
higher by about one order of magnitude than when the heater is
provided over the entire substrate surface. Where a resistance film
10 .mu.m wide is used, the injection power is reduced by one sixth.
The injection power is not strictly proportionate to the ratio of
the resistance film width of 10 .mu.m to the diode width of 400
.mu.m because the heat dissipates crosswise within the
substrate.
As described, depositing the heater on the surface of the diode
surface makes the response time of temperature change around the
active layer shorter than the conventional setup of the heater
being located at the back of the substrate. In addition, reducing
the heater width enhances the energy efficiency for temperature
control and further promotes temperature change around the active
layer.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view of a wavelength tunable laser diode
embodying the invention;
FIG. 2a is a graphic representation of temperature rise versus
response time measurements taken in two cases: one in which the
semiconductor substrate is heated on the surface thereof and the
other from the back thereof; and
FIG. 2b is a graphic representation of temperature rise versus
response time measurements taken in two cases: one in which the
entire surface of the substrate is heated by a resistance film, and
the other in which part of the substrate surface is heated by the
resistance film.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS:
FIG. 1 is a schematic view showing a wavelength tunable laser diode
practiced as one preferred embodiment of the invention. Referring
to FIG. 1, an active layer 1 and a p-type InP layer 2 are formed on
an n-type InP substrate 3 using the known metal-organic vapor phase
deposition epitaxial growth technique. The active layer 1 has a
multiple quantum well structure of InGaAs/InGaAsP. After this, the
active layer 1 is subjected to etching so that an active layer
strip about 1 .mu.m wide is left and the rest removed. An Fe-doped
high resistance layer 10 is formed where the active layer 1 was
removed so that a carrier is injected only into the remaining
active layer 1. Above the active layer 1 is formed an SiO.sub.2
film 5 having a window above the remaining active layer strip.
Above the film 5 is a p-type electrode 6 made principally of gold
(Au) and into which a carrier is injected for laser oscillation.
The manufacturing stages so far are the same as those of
conventional laser diode manufacture. To implement a wavelength
tunable laser having a single wavelength and a narrow spectral line
width, there is provided a distributed feedback laser diode
structure under the active layer 1 in the optical axis direction
(in the direction of depth in FIG. 1). This distributed feedback
laser diode structure has a grating 0.24 .mu.m in pitch.
In FIG. 1, on the right-hand side of the p-type electrode 6 are
formed electrodes 9-1 and 9-2 in an insular fashion. The two
electrodes 9-1 and 9-2 are connected to both ends of a resistance
film 8 made of a Pt film. Thereafter, an SiO.sub.2 layer 7 about 20
.mu.m wide is deposited on the active layer 1. On top of the layer
7 is formed the Pt film as the resistance film 8 which is 0.7 .mu.m
thick and 10 .mu.m wide. As mentioned, the two ends of the Pt film
8 are in contact with the previously formed electrodes 9-1 and 9-2.
Finally, an n-type electrode 4 is deposited over the entire back of
the substrate 3, the electrode 4 being made primarily of gold
(Au).
After producing a specimen 200 .mu.m thick and 400 .mu.m wide in
the above-described manner, the inventors attached the back of the
specimen to a heat radiation block that could be controlled in
temperature using a Peltier device, and tested the specimen for
characteristic evaluation. During the test, the block was kept to a
constant 20.degree. C. When a carrier of 250 mA was injected into
the p-type electrode 6, a laser beam 1.540 .mu.m in wavelength, 1
MHz in spectral line width and 20 mW in optical output power was
obtained. When a current of 400 mA was injected into the resistance
film 8 via the electrodes 9-1 and 9-2, the temperature rose by
20.degree. C. 10 .mu.s later, with the wavelength shifted by 2 nm
to reach 1.542 .mu.m. After a period of 100 .mu.s, the temperature
rose by 40.degree. C. and the wavelength moved by 4 nm to the long
wave side, reaching 1.544 .mu. m. Within one millisecond after
this, feedback control by the receiver produced a laser beam of
stable oscillation at 1.544 .mu.m. At this point, the optical
output power was 10 mW or higher and the spectral line width 10 MHz
or less.
Although one specific embodiment of the invention has been
described, the embodiment is not limitative of the invention and
may be modified in various ways. For example, the resistance film
used as the temperature variable heater may be replaced by an
alternative temperature variable heater as long as the alternative
heater, located close to and above the active layer, is wider than
the active layer and smaller than the laser diode surface area. One
such alternative heater is a Peltier device. Where a resistance
film constitutes the temperature variable heater, the active layer
may be shifted a little off the center and the film may be formed
on the side of the active layer. This alternative construction
reduces the device size and simplifies the manufacturing process at
a tolerable cost of a slight drop in the thermal response
characteristic.
As described and according to the invention, the temperature
variable heater is provided close to the active layer of the laser
diode. This structure widens the tunable range of the laser diode
and improves the thermal response characteristic thereof.
Illustratively, the embodiment depicted above provides a tunable
range of 4 nm within one microsecond. With conventional
temperature-controlled laser diodes, the Peltier device is attached
to the electrode at the back of the semiconductor substrate not
directly but through a heat sink metal block. This has further
prolonged the response time of temperature change. The time delay
involved entails an overshoot phenomenon. This means that it takes
nearly one second to acquire a laser beam of sufficiently stable
oscillation. By contrast, the present invention not only improves
the response time of temperature change but also stabilizes quickly
the temperature near the active layer of the device through the
improved response.
Where a resistance film is used as the heater, reducing the film
width boosts the injection energy density. This further enhances
the response time and energy efficiency, which permits easer
control. For example, the above-described embodiment is a
wavelength tunable laser diode that provides a laser beam 1.54
.mu.m in wavelength, 1 MHz in spectral line width and 20 mW in
optical output power at a response time of one millisecond or less
over a tunable range of 4 nm.
As many apparently different embodiments of this invention may be
made without departing from the spirit and scope thereof, it is to
be understood that the invention is not limited to the specific
embodiments thereof except as defined in the appended claims.
* * * * *